The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be a few tens of nanometers (nm), or less. These inspection tools must operate at high speed in order to inspect a large fraction, or even 100%, of the area of a photomask, reticle, or wafer, in a short period of time. For example, inspection time may be one hour or less for inspection during production or, at most, a few hours for R&D or troubleshooting. In order to inspect so quickly, inspection tools use pixel or spot sizes larger than the dimensions of the defect or particle of interest, and detect just a small change in signal caused by a defect or particle. High speed inspection is most commonly performed in production using inspection tools operating with UV light. Inspection in R&D may be performed with UV light or with electrons.
Once a defect or particle has been found by high speed inspection, it is often necessary to make a higher resolution image and/or to perform material analysis to determine the origin or type of the particle or defect. This process is commonly called review. Review is usually performed with a scanning electron microscope (SEM). Review SEMs used in semiconductor manufacturing are typically required to review many thousands of potential defects or particles per day and may have, at most, a few seconds per target for review.
Electron microscopes need an electron source to generate an electron beam directed toward a sample. Electron sources can be divided into two broad groups: thermionic sources and field emission sources. Thermionic sources are the most common commercially available electron emitters, and are usually made of tungsten or lanthanum hexaboride (LaB6). In thermionic emission, electrons are boiled off the material surface when the electron thermal energy is high enough to overcome the surface potential barrier. Thermionic emitters typically require elevated temperatures (>1300 K) to operate, and have several drawbacks such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The demand for more efficient electron sources has driven the research and development of Schottky emitters and cold electron sources such as electron field emitters.
In the Schottky emitters, thermionic emission is enhanced by effective potential barrier lowering due to the image charge effect under an applied external electric field. Schottky emitters are typically made of a tungsten wire having a tip coated with a layer of zirconium oxide (ZrOX), which exhibits a much lower work function (˜2.9 eV). Thermally-assisted Schottky emitters need to be operated at high temperature (>1000 K) and high vacuum (˜10−9 mbar), and have wider than desirable electron emission energy spread due to the high operating temperature. An electron source with lower energy spread, higher brightness (radiance), and higher current density than Schottky emitters is desirable for semiconductor wafer and mask inspection, review, and lithography as it will enable faster and more cost effective inspection, review, and lithography.
Cold electron sources, particularly electron field emitters, have been used in field emission displays, gas ionizers, x-ray sources, electron-beam lithography, and electron microscopes, among other applications. Field emission takes place when the applied electric field is high enough to reduce the potential barrier on the tip-vacuum interface so that electrons can tunnel through this barrier at a temperature close to room temperature (i.e., quantum-mechanical tunneling). A typical field-emitter consists of a conical emitter tip with a circular gate aperture. A potential difference is established across the emitter cathode, gate, and anode under an applied external field, resulting in high electric field at the surface of the tip. Electrons tunnel through the narrow surface barrier and travel toward an anode, which is biased at a more positive potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitters.
Field emitters, because they can operate near room temperature, have lower energy spread than Schottky and thermionic emitters, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of a field emitter is less stable as contaminants can easily stick to the tip of the emitter and raise its work function, which will lower the brightness and current. Periodic flashing (i.e., temporarily raising the tip temperature) is required to remove those contaminants. The instrument is not available for operation while the tip is being flashed. In the semiconductor industry, instruments are required to operate continuously and stably without interruption for long periods, so Schottky emitters are usually used in preference to cold field emitters.
Early efforts have been concentrated on developing metallic field emitters. Among others, Spindt-type molybdenum field emitters are perhaps the most well-known metallic field emitters because molybdenum has a low resistivity (53.4 nΩ·m at 20° C.) and a high melting point (2896 K). Nevertheless, metallic emitters suffer from several disadvantages such as lack of uniformity due to metal deposition techniques, and, more severely, the degradation in emission current, mainly due to oxidation.
With the advent of modern semiconductor fabrication technology, there has been investigation of semiconductor field emitters, particularly silicon field emitters. Single-crystal (monocrystalline) silicon is an attractive material for field emitters. Silicon crystals can be grown with very high purity and very few crystal defects. The conductivity of silicon can be altered by doping and/or applying a voltage. More importantly, silicon has a well-developed technology base.
The structure of a typical prior-art silicon field emitter is shown in FIG. 6. A silicon substrate 61 is doped with impurities and can be either n-type or p-type doped. The cone-shaped emitter 64 is formed on the silicon substrate 61, with an optional gate layer 67 attached to a dielectric layer 66, which includes one or more insulating layers. The optional gate layer 67 controls and extracts the emission current. A third electrode (i.e., the anode (not shown)), faces the gate layer 67 and is separated at a large distance, on the order of hundreds of microns, from the cathode. This is the typical silicon field emitter triode configuration. Note that without the gate layer 67, the field emitter can be used as a diode. Quantum tunneling of electrons takes place when a bias voltage is applied across the emitter structure. A large electrical field is generated on the surface of the emitter tip, and electrons are emitted from the tip.
Even though silicon field emitters have shown promise in recent years, they are not yet commercially available. One serious problem with the use of silicon to form field emitters is that silicon is quite reactive, and can be contaminated within hours, even at pressures around 10−10 torr. Silicon very readily forms a native oxide on its surface. Even in a vacuum, a native oxide will eventually form as the small amounts of oxygen and water present in the vacuum will react with the surface of the silicon. The interface between silicon and silicon dioxide has defects (due to dangling bonds) where the probability of an electron recombination is very high. Furthermore, the band gap of silicon dioxide is large (about 9 eV) creating an additional barrier higher than the work function that an electron has to overcome in order to escape, even if the oxide is very thin. For example, native oxide on a very smooth silicon surface is typically about 2 nm thick. In some circumstances, oxidation can also change the shape of the field emitters. These aforementioned problems may result in low brightness and current as well as poor stability of emission, the lack of reliability, scalability and uniformity, and have hindered the commercial use of silicon field emitters.
Research effort has been expanded in looking for surface treatments and coatings for field emitters to improve their performance for lower turn-on voltages, higher emission current densities, lower noise, and improved stability. These treatments may include coating the emitter tips with refractory metals, silicides, carbides, and diamond, etc. However, these coating materials are usually limited by the fabrication process in forming smooth and uniform coating surfaces, and/or are often affected by the oxide layer formed on the coating surfaces, creating an additional energy barrier. For these reasons, coated silicon field emitters have not become yet practical as cold electron sources.
What is therefore needed is an electron source that overcomes some, or all, of the limitations of the prior art.